An Introduction to TBW - Terabytes Written

January 2, 2019 Gibson Ming-Dar Chen, UDinfo

SSDs (Solid-State Drives) have been widely used in computer related territories for a while. However, the life of a SSD and the total SSD data amount that can be written is still an important topic to discuss.  Terabytes Written, or TBW, is the total amount of terabytes data that a SSD can write in its lifetime.

The TBW formula is as follows: 

For example, a MLC 128GB SSD, which NAND has 3000 times P/E cycles, should be able to provide 128GB*3000 to equal 384TB data written theoretically. However, the NAND is composed of pages, the minimum unit for program, and blocks (composed of hundreds of pages), the minimum unit for erasing.

When the host writes files into a SSD, the total bytes written into the NAND usually are different from the data sent from the host. It is due to the size of each file being different from the NAND page, the alignment of data and the beginning of each page, and algorithms from SSD controller, such as data compression, which comprises the WAF, is usually not equal to 1. For example, if the system wants to write a 512-byte file into the SSD, the NAND needs to write the minimum size, the size of a page, such as 8 kbytes, and it makes the WAF= 8K/512=16. If the WAF is too big, it will make the TBW very small. However, every host and PC user might have different user frequency and habits, so the input data size combinations are quite different. 

Therefore, JEDEC set up a standard, the JESD219 as SSD Endurance Workloads, for the workload of SSD. It includes Client and Enterprise workloads for different user scenarios.

The Enterprise workload has many small files compared to the Client workload, which makes the WAF of Enterprise much higher.

For example, the WAF for the Client workload for most of SSD is around 2 to 4 and the one for Enterprise varies at around 10. 

For embedded applications, if the workload of the end user is different from these standards, a customized TBW calculation can be provided as a service.

The higher the TBW, the longer the SSD can be used.

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